The present invention refers to a method and an apparatus for processing bed material in a fluidized bed reactor, having a reactor chamber, with side walls defining the interior of the reactor chamber and a fluidized bed of solid particles in the lower part of the reactor chamber.
In some fluidized bed processes it may be desirable to gather a portion of the particles forming the bed material for separate processing, e.g. cooling, before further processing that portion of the bed material in the reactor chamber. It has been suggested to discharge solid particles from a reactor chamber into a separate processing chamber and after processing thereof (e.g. cooling), to recycle the particles back into the reactor chamber.
In some processes it is advantageous to process only particles of a certain size range. Particles of a small size e.g. suitable for heat recovery are, however, due to fractionation in the lower part of the reactor chamber, often mixed with large particles or other large objects. This tends to prevent optimal heat recovery, heat transfer being more efficient in a bed of small particles. Large objects also tend to cause problems by e.g. clogging the heat transfer surfaces and also by causing mechanical damage.
Further problems to be solved when processing particles in a separate processing chamber arise with the recirculation of the material from the reactor chamber into the processing chamber and back into the reactor chamber. A pressure difference prevailing between the lower part of the reactor chamber and the processing chamber forces particles to flow from the reactor chamber into the processing chamber. But the recycling of the particles back into the reactor chamber requires mechanical or other transporting means to overcome the pressure difference.
The present invention provides an improved method and apparatus for processing bed material in a separate processing chamber connected to the reactor chamber of a fluidized bed reactor. The invention particularly provides an improved method and apparatus for recycling bed material between the reactor chamber and the separate processing chamber. The present invention employs a fluidized bed reactor for recycling bed material of a size smaller than a predetermined size through an external processing chamber, and recovers heat from bed material in an external processing chamber.
The method according to the invention for processing bed material in a fluidized bed reactor chamber, utilizes an outlet duct from the reactor, connected to a lifting chamber, in turn connected to a process chamber. The method comprises the steps of: (a) Discharging solid particles from the lower part of the reactor chamber at a first level of the fluidized bed through the outlet duct into the lifting chamber. (b) Pneumatically conveying solid particles with a conveying gas, as a particle suspension, upwardly in the lifting chamber, to a second level higher than the first level and into a processing chamber. (c) Forming a fluidized or fixed bed of solid particles in the processing chamber. (d) Processing the solid particles in the processing chamber. And, (e) recycling processed solid particles with conveying or fluidizing gas from the processing chamber into the reactor chamber, at a third level, higher than the first level.
Processed solid particles may additionally be recycled from the processing chamber into the reactor chamber at a fourth level, lower than the third level.
Exemplary apparatus for processing bed material in a fluidized bed reactor according to the invention comprises: A lifting chamber connected to the lower part of the reactor chamber. An outlet duct connecting the reactor chamber with the lifting chamber, the outlet duct being disposed at a first level in the reactor chamber, for discharging solid particles from the reactor chamber into the lifting chamber. A processing chamber connected to the lifting chamber, the processing chamber including a bed of solid particles. A connector duct connecting the upper part of the lifting chamber with the processing chamber. Means for introducing conveying gas (such as air) into the lifting chamber for pneumatically conveying solid particles from the lifting chamber through the duct into the lifting chamber. Means for processing solid particles in the processing chamber. And, a first inlet duct connecting the processing chamber with the reactor chamber, the first inlet duct disposed at a second level, higher than the first level, for recycling processed solid particles with conveying or fluidizing gas from the processing chamber into the reactor chamber.
The lifting chamber and the processing chamber are preferably disposed adjacent the reactor chamber to form a compact unit. A portion of a side wall of the reactor chamber may divide the lifting and processing chambers from the reactor chamber. Similarly the lifting chamber and the processing chamber may be constructed as a unit, and separated by a partition wall.
The first outlet duct may be disposed in the refractory lined lower part of the reactor chamber, in the side wall e.g. up to a 1000 mm level above the grid, or in an extension chamber below the grid level. The outlet duct is preferably disposed in the reactor chamber at a dense bed zone level, having &gt;100 kg solids / m.sup.3 gas, preferably 500-1500 kg/m.sup.3. The first inlet duct is then preferably disposed at a level 500-5000 mm above the outlet duct.
The pressure prevailing in a fluidized bed reactor varies with the height of the bed. At a first low level in the dense bed zone of the reactor chamber the pressure p.sub.1 may be much higher than the pressure P.sub.2 at a higher second level at a less dense bed zone.
The present invention provides an easy way of recycling solid particles from a high pressure p.sub.1 at a dense bed zone into a processing chamber and back into the reactor chamber without the need of mechanical or other complicated conveyors.
Solid particles are discharged from a dense bed zone at a first level in the reactor chamber by pressure difference into the lifting chamber, the pressure P.sub.3 in the lifting chamber being lower than the pressure p.sub.1 in the dense bed zone of the reactor chamber.
A nonmechanical seal, such as a gill seal or an L-valve, may be disposed in the outlet duct in order to prevent gases from flowing from the lifting chamber into the reactor chamber and to control solids flow into the lifting chamber. A gill seal is formed by several narrow slots, i.e. slot formed openings, arranged on top of each other. The slots are preferably slightly inclined from the horizontal, each slot forming a nonmechanical seal. The amount of bed material flowing into the lifting chamber may be controlled by fluidizing air nozzles provided at the nonmechanical seal.
In the lifting chamber the solid particles are lifted as a particle suspension by pneumatic conveyance to a second, higher, level. At the second level in the reactor chamber the pressure p.sub.2 is considerably lower than the pressure p.sub.1 in the dense bed zone at the first, lower, level. The particle suspension is further conveyed at this second level into the processing chamber.
At the second level only a small pressure difference prevails between the reactor chamber and the processing chamber. The conveying gas forced to flow through the first inlet duct into the reactor chamber thus effects a pressure difference which prevents gases from flowing backwards from the reactor chamber into the processing chamber. A portion of the solid particles conveyed from the lifting chamber into the processing chamber may flow with the conveying gas through the first inlet duct directly back into the reactor chamber.
In the processing chamber solid particles are partially separated from the particle suspension due to the change in direction of flow and due to particle strand formation. Separated particles form a fluidized or fixed bed of solid particles in the processing chamber. Fluidizing gas and particles carried by the gas flow through the first inlet duct into the reactor chamber.
A second inlet duct may be disposed at a lower level in the processing chamber for recycling mainly particles from the fluidized or fixed bed by pressure or gravity into the reactor chamber. The pressure in the processing chamber varies with the height and density of the bed as does the pressure in the reactor chamber. The pressure p.sub.4 in the processing chamber may be held at a higher level than the pressure p.sub.2 in the reactor chamber at corresponding levels by increasing the bed density or height of the bed in the processing chamber. The pressure p4 may also be held higher than the pressure p2 by providing the processing chamber at a level high enough so that the pressure p2 is smaller than pressure p4.
According to the present invention it is possible to pneumatically lift solid particles in the lifting chamber to a considerably higher level, if necessary, in order to overcome the pressure in the reactor chamber. The pressure difference between the processing chamber and the reactor chamber forces solid particles to flow from the processing chamber into the reactor chamber and prevents gases from flowing from the reactor chamber into the processing chamber.
The second inlet may be disposed at substantially the same level as the outlet duct. However, then the pressure in the fluidized or fixed bed in the processing chamber has to be increased by increasing the density of the bed or by increasing the height of the bed to overcome the pressure in the reactor chamber.
The present invention utilizes these and other pressure differences for recycling solid material between the reactor chamber, lifting chamber and processing chamber. Especially utilized are pressure differences between the reactor chamber and lifting chamber on one hand, and between the reactor chamber and processing chamber on the other hand.
The bed material to be processed in the processing chamber may have been classified before being introduced into the reactor chamber, by providing a barrier or classifying wall across the inlet of the outlet duct. The barrier wall may be a partition wall having openings or slots preventing particles of a size larger than a predetermined size to flow therethrough into the outlet duct.
Alternatively the fractioning of the bed material may take place in the lifting chamber. Fluidizing air nozzles arranged in the bottom of the lifting chamber may be used to control the size of particles allowed to flow upwardly and through the duct into the processing chamber. Small particles are pneumatically transported into the processing chamber whereas larger objects are discharged through the bottom of the lifting chamber or are mechanically recycled back into the reactor chamber.
The lifting chamber may be used for discharging ash from a fluidized bed combustor. Ash may then be classified in the lifting chamber. Large inert ash particles are discharged from the system and fine flyash is transported into the processing chamber for further processing.
According to the invention only a controlled fraction of solid particles of a predetermined size may be allowed to flow into the processing chamber. The fractioning may take place when discharging particles from the reactor chamber, or when lifting solid particles upwardly in the lifting chamber. The fine solid particles are then advantageously further processed, e.g. cooled in a heat exchanger, in the processing chamber, without large objects causing trouble.
The present invention may be utilized for recovering heat from solid particles by disposing heat transfer surfaces in the processing chamber. The heat transfer may be controlled by fluidizing air introduced into the processing chamber. The processing chamber is especially suitable for generating high temperature steam, as the gas atmosphere therein contains only very small amounts, if any, of corrosive gaseous compounds.
In hot processes the wall between the reactor chamber and the housing including the lifting chamber and the processing chamber (as well as other walls in the chamber) may be constructed of water tube panels. The tube panels may be refractory lined and bent to form openings between them. 0n the other hand if classification of material is needed an outlet duct having a barrier wall is easily disposed in a refractory lined tube panel by making furrows in the refractory lining between the adjacent tubes and revealing the fin connecting the two tubes. Round holes or slots of desired size can thereafter be made in the fins.
A gill seal may be inserted in a refractory lined tube panel by bending adjacent tubes apart enough to provide space for the gill seal arrangement. The openings may be formed in different modes.
The present invention provides an improved method and apparatus for processing bed material in an external processing chamber, while in an improved way circulating a desired fraction of the material from the reactor chamber through the processing chamber and back into the reactor chamber.
The present invention thereby provides a method for processing material in an external processing chamber in a fluidized or fixed bed of particles, at a different temperature, different fluidization conditions, or different gas atmospheres.